(1) Technical Field
The present invention relates to an alkali-free glass and, in particular, to an alkali-free glass for use as a light transparent glass substrate for a display and the like as well as a method of producing the same.
(2) Prior Art
Conventionally, an alkali-free glass has been used as a light transparent glass substrate for a liquid crystal display and the like. The alkali-free glass for use in such a display is required to be free from bubbles resulting in a display defect, in addition to various characteristics such as heat resistance and chemical resistance.
As such an alkali-free glass, various kinds of glasses have been proposed. In Japanese Unexamined Patent Publication (JP-A) No. 63-74935, the present applicant proposes an SiO2xe2x80x94Al2O3xe2x80x94B2O3xe2x80x94CaOxe2x80x94BaO alkali-free glass.
In order to obtain a glass without bubbles, it is important to select a fining agent capable of generating a fining gas in a temperature range from the temperature of vitrification to the temperature of homogenization melting. Specifically, the fining of a glass is carried out by expelling a gas generating during the vitrification from a glass melt with the help of the fining gas and further removing very small remaining bubbles after they are enlarged by the fining gas generated again during the homogenization melting to float up.
In the meanwhile, the alkali-free glass for use as a glass substrate for a liquid crystal display is high in viscosity of the glass melt so that a melting process is carried out at a high temperature in comparison with a glass containing an alkali component. In the alkali-free glass of the type, the vitrification generally occurs at 1200-1300xc2x0 C. while the fining and the homogenization are performed at a high temperature of 1400xc2x0 C. or more. Under the circumstances, As2O3 capable of producing the fining gas in a wide temperature range (on the order of 1200-1600xc2x0 C.) is widely used as the fining agent.
However, As2O3 is highly toxic and may possibly cause environmental pollution during a manufacturing process of the glass and during disposal of a waste glass. In this reason, the use of As2O3 is being limited.
It is an object of the present invention to provide an alkali-free glass which does not use As2O3 as a fining agent and which is free from bubbles resulting in a display defect, and to provide a method of producing the same.
As a result of various experiments, the present inventors have found out that the above-mentioned object can be achieved by using, as a fining agent, a combination of SnO2 and chloride instead of As2O3, and propose the present invention.
Specifically, an alkali-free glass according to one aspect of the present invention is characterized in that the glass essentially consists of, by weight percent, 40-70% SiO2, 6-25% Al2O3, 5-20% B2O3, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, 0-10% ZnO, 0.05-2% SnO2, and 0.005-1% Cl2, and that the glass substantially contains no alkali metal oxide.
A method of producing an alkali-free glass according to another aspect of the present invention comprises preparation of a glass batch mixed so as to obtain a glass which essentially consists of, by weight percent, 40-70% SiO2, 6-25% Al2O3, 5-20% B2O3,0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO and which substantially contains no alkali metal oxide, melting of the glass batch, and forming of the glass melt, and is characterized by addition of, as a fining agent, 0.05-2 weight % SnO2 and 0.01-2 weight % chloride in terms Of Cl2 into the glass batch.
Prior to description of embodiments of the present invention, the principle of the present invention will be described.
SnO2 used in the present invention generates a large amount of fining gas as a result of variation in valence of Sn ions, which takes place in a temperature range of 1400xc2x0 C. or more. On the other hand, chloride is decomposed and volatilized in a temperature range not lower than 1200xc2x0 C. to generate the fining gas (chroline gas, etc.). As the temperature is elevated, decomposition and volatilization become very active to generate a large amount of the fining gas.
Accordingly, the use of a combination of SnO2 and chloride as the fining agent provides a high fining effect in a wide temperature range from a comparatively low temperature in which the vitrification occurs to a high temperature in which homogenization melting occurs.
As a result, it is possible to obtain the alkali-free glass without bubbles resulting in a display defect.
Next, description will be made about a method of producing the alkali-free glass according to the present invention.
At first, a glass batch is prepared so as to obtain a glass having a desired composition. Description will hereafter be made about the glass composition range and the reason why each content is so defined.
SiO2 is a component serving as a network of the glass. The content is 40-70%, preferably, 45-65%. When the content of SiO2 is less than 40%, chemical resistance is degraded and a strain point is lowered so that heat resistance is degraded. On the other hand, when the content is more than 70%, high-temperature viscosity is increased so that the meltability is deteriorated and devitrified substances of cristobalite readily precipitate.
Al2O3 is a component to enhance the heat resistance and devitrification resistance of the glass. The content is 6-25%, preferably, 10-20%. When the content of Al2O3 is less than 6%, the devitrification temperature remarkably rises so that the devitrification is readily caused to occur in the glass. When the content is more than 25%, acid resistance, particularly, buffered-hydrofluoric-acid resistance is degraded so that the cloudness is readily caused to occur on the surface of the glass substrate.
B2O3 is a component serving as a flux to lower the viscosity and to facilitate melting. The content is 5-20%, preferably, 6-15%. When the content of B2O3 is less than 5%, the effect as the flux is insufficient. When the content is more than 20%, the hydrochloric acid resistance is degraded and the strain point is lowered so that the heat resistance is degraded.
SnO2 and chloride used as the fining agent in the present invention exhibits a slightly inferior fining effect during vitrification at a low temperature in comparison with the fining effect during the homogenization melting at a high temperature. In case where the fining effect is insufficient in such a low temperature range, a melting temperature is elevated to a higher temperature. However, if it is difficult to elevate the melting temperature, B2O3 is preferably contained to a ratio of 8.4% or more.
The reason is as follows. When at least 8.4% B2O3 is added, it works strongly as a flux to readily expel the gas generating during the vitrification. This complements the fining effect of SnO2 and chloride in the low temperature range.
MgO is a component for decreasing the high-temperature viscosity without lowering the strain point so as to facilitate melting of the glass. The content is 0-10%, preferably, 0-7%. When the content of MgO is more than 10%, the buffered-hydrofluoric-acid resistance of the glass is significantly degraded.
CaO has a function similar to that of MgO. The content is 0-15%, preferably, 0-10%. When the content of CaO is more than 15%, the buffered-hydrofluoric-acid resistance of the glass is significantly degraded.
BaO is a component to improve the chemical resistance of the glass and to improve the devitrification resistance. The content is 0-30%, preferably, 0-20%. This is because, when the content of BaO is more than 30%, the strain point is lowered so that the heat resistance is degraded.
SrO has an effect similar to that of BaO. The content is 0-10%, preferably, 0-7%. The reason is that the content of SrO more than 10% is unfavorable because the devitrification is increased.
ZnO is a component to improve the buffered-hydrofluoric-acid resistance and the devitrification resistance. The content is 0-10%, preferably, 0-7%. This is because, when the content of ZnO is more than 10%, the glass tends to be devitrified and the strain point is lowered so that the heat resistance cannot be obtained.
When the total content of MgO, CaO, BaO, SrO, and ZnO is less than 5%, the high-temperature viscosity is increased so that the meltability is degraded and the glass is readily devitrified. On the other hand, the total content more than 30% is unfavorable because the heat resistance and the acid resistance are degraded.
In addition to the above-described components, it is possible to add ZrO2, TiO2, Fe2O3, and the like up to 5% in total. It is preferable not to add Sb2O3 widely used as a substitute fining agent for As2O3 because it is toxic although not so strong as As2O3.
Next, SnO2 and chloride are added to the glass batch.
The amount of SnO2 to be added is 0.05-2weight % with respect to the glass batch as 100weight %. This is because, when the content of SnO2 is less than 0.05%, it is difficult to remove the bubbles remaining in the glass melt during the homogenization melting. On the other hand, when the content is more than 2%, the glass tends to be devitrified.
The amount of chloride to be added is 0.01-2 weight % in terms of Cl2. This is because, when the content of chloride is less than 0.01%, it is difficult to expel the gas generating during the vitrification and to remove the bubbles remaining in the glass melt during the homogenization melting. On the other hand, when the content of chloride is more than 2%, the amount of volatilization is excessive so that the glass tends to be deteriorated. As chloride, use can be made of BaCl2, CaCl2, and the like.
Then, the glass batch thus prepared is melted. As the glass batch is heated, the vitrification at first takes place. At this time, the fining gas such as chlorine gas is generated as a result of decomposition of chloride so that the gas generating during the vitrification is expelled from the glass melt. Furthermore, during the homogenization melting at a higher temperature, oxygen gas generates as a result of chemical reaction following the variation in valence of SnO2. In addition, chloride is decomposed and volatilized to produce the gas. Thus, very small bubbles remaining in the glass melt are removed.
Thereafter, the glass melt is formed into a desired shape. If the glass of the present invention is used as a display, the glass melt is formed into a thin plate shape by the use of a fusion process, a downdraw process, a float process, a roll-out process, and the like.
In the above-mentioned manner, it is possible to obtain the alkali-free glass which essentially consists of, by weight percent, 40-70% SiO2, 6-25% Al2O3, 5-20% B2O3, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, 0-10% ZnO, 0.05-2% SnO2, and 0.005-1% Cl2, and which substantially contains no alkali metal oxide.